This invention relates generally to logging tools and more particularly to a logging tool induction coil form having a coating layer formed thereon, which comprises a material more easily formed to tight tolerances than silicon nitride.
Measurements from induction logging tools used in oil and gas well data logging applications are very sensitive to changes in the position or the diameter of either the receiver or transmitter coils. Ideally, during logging the receiver and transmitter coils do not move relative to one another. Furthermore, ideally, the diameters of the respective coils do not vary during logging. Appropriate material selection has been the approach to control the movement component associated with changes in the diameter of the tool. More specifically, materials have been selected, which cause the dimensions of all the parts of the induction tool to vary as little as possible even as the temperature of the tool varies in a wide range that can go from −40° F. during calibration to as high as 500° F. in the deepest wells. To reduce dimensional changes due to thermal expansion, low thermal expansion materials are normally used. The exceptionally low thermal expansion coefficient of silicon nitride and its good resistance to fracture make this material the common choice to reduce the axial movement due to thermal expansion.
The precise axial position of the receiver and transmitter coils on the induction coil form is also critical in obtaining accurate measurements. To ensure correct placement of the coils on the induction coil form, grooves are machined into the outer surface of the form. However, silicon nitride is a very hard material that needs to be ground to very tight tolerances so as to achieve the desired electromagnetic response of the instrument. Thus, the cost of precision grinding the induction coil form is high and constitutes the largest part of the cost of the finished product. None of the prior solutions have offered a cost efficient way to grind the grooves onto the outer surface of silicon nitride induction coil forms.